When an automobile is initially assembled at the factory, all the parts have a predefined manufacturing tolerance and therefore the interface between various moving parts can be predicted. However, after an engine has been utilized for some time, these parts wear and go out of tolerance. This wear usually results in failure of the engine.
When remanufacturing an engine, it is necessary to replace worn parts. However, the replacement parts typically are dimensioned to accommodate the overall wear of the engine. For example, when a valve guide is worn, it becomes somewhat enlarged along the central axis thereof. If a valve stem having the original manufacturer's tolerance were installed into the remanufactured engine, the space between the exterior surface of the valve stem and the interior surface of the valve guide would exceed acceptable tolerances. Therefore, some accommodation must be made for the various wearing surfaces and the cooperating relationship therebetween in the remanufactured engine.
Valves in particular on the remanufactured engine present some unique problems in that failure to meet acceptable tolerances can result in poor heat transfer, excessive oil consumption, etc. There are three key items of consideration with respect to the valve in a remanufactured engine. First, it is essential to ensure that the valve in a remanufactured engine properly seats in the head and that the valve seat itself is ground to acceptable tolerances to provide a smooth mating surface for the valve. The valve head must have a smooth mating surface to provide proper seating. Second, the valve stem and the valve guide must interface such that there is approximately between a 0.0015 to 0.003 inch clearance between the exterior surface of the valve stem and the interior surface of the valve guide. Further, it is necessary that heat be transferred from the valve guide to the head. Third, the keeper groove at the upper end of the valve stem defines the securing position for compressing the valve spring. If the position of the keeper groove relative to the exterior of the head is altered from that of the original manufactured engine, this will result in the valve spring having too little compression or too much compression.
When regrinding a valve seat, it is important to remove a sufficient amount of the seating surface to provide a smooth polished surface. Depending upon the degradation of this surface, the seat may have to be lowered into the head too great a distance. Since this would result in the new or remanufactured valve sitting too far into the head and also result in removal of too much of the seat, the solution would be to remove a larger portion of the seat and replace it with a cast iron donut insert. This is a relatively expensive procedure.
When a valve guide wears, prior procedures have required boring out the valve guide and inserting a liner which has an inner diameter sized to provide the appropriate tolerances with the valve stem. Of course, the appropriate valve stem must be selected to be mated with the new liner. The problem with utilizing some types of liners or inserts is that the liner is typically pressed into the bored out valve guide. Although this press fit is relatively tight, heat transfer through the new liner is somewhat impeded. This is due to the fact that the interface between the exterior surface of the liner and the interior surface of the bored out valve guide is poor.
To accommodate variations in the height of the keeper groove at the end of the valve stem above the engine head, shims are typically utilized. One purpose for shims is to account for the "deterioration" or set of the valve spring and thus provide more spring tension. In addition, the shims also account for grinding of the valve seat wherein the head of the valve will recede into the valve seat. This directly translates to the keeper groove rising above the engine head.
One problem that exists at present with remanufacturing engines and especially with the valves is that generally there is no standardized value for rebuilding and only standard valves are available. Therefore, it is necessary to utilize the above rebuilding techniques with unique tooling for each engine in many cases to accommodate these various standardized valves.